If this is the first declaration for <sym-font> then a new
symbol font with this name is created (i.e. this identifier is assigned
to a new TeX math group).

The arguments <encoding> <family> <series> <shape> are used to
set, or reset. the default values for this symbol font in all
math versions; if required, these must be further reset later for
a particular math version by a \SetSymbolFont command.

Checks that <encoding> is a declared encoding scheme.

For example, the following sets up the first four standard math symbol
fonts:

This declaration should be used in preference to
\DeclareMathAlphabet and \SetMathAlphabet when a math alphabet is
the same as a symbol font; this is because it makes better use of the
limited number (only 16) of TeX's math groups.

Note that, whereas a TeX math group is allocated to each symbol font
when it is first declared, a math alphabet uses a TeX math group
only when its command is used within a math formula.